Mitochondrial neurogastrointestinal encephalomyopathy (MNGIE) is a fatal autosomal recessive disease caused by mutations in the TYMP gene encoding thymidine phosphorylase (TP). The disease typically begins in childhood or early adulthood causes ptosis, ophthalmoplegia, severe gastrointestinal dysmotility leading to cachexia, peripheral neuropathy, leukoencephalopathy, and alterations of mitochondrial DNA (mtDNA). Most patients die by age 40. Through genetic linkage studies, we identified the causative gene and demonstrated that TYMP mutations cause TP deficiency, which leads to dramatic elevations of thymidine and deoxyuridine in plasma and tissues of patients. In addition, we identified multiple deletions, depletion, and site-specific point mutations of mtDNA in patients' tissues and cells. Based on these findings, we hypothesize that toxic accumulations of thymidine and deoxyuridine nucleosides cause imbalances of mitochondrial deoxynucleotide pools leading to mtDNA instability. Other investigators have demonstrated that TP-expressing human cells cultured in high concentrations of thymidine have unbalanced mitochondrial nucleotide pools and develop partial depletion or multiple deletions of mtDNA. We have generated a TP and uridine phosphorylase (UP) double-knockout (TP-/-UP-/-) mouse model of MNGIE. The TP-/-UP-/- mice show loss of TP activity, elevated levels of thymidine and deoxyuridine in tissues, mitochondrial nucleotide pool imbalances, and leukoencephalopathy with mtDNA depletion and respiratory chain defect. In this application, we propose to further test our hypotheses that: 1) TP deficiency causes toxic accumulation of thymidine, that, in turn, lead to nucleotide pool imbalances and mtDNA instability and 2) restoration of TP activity by bone marrow transplantation or nanoparticle enzyme replacement will be therapeutic. Three specific aims are proposed. Aim 1: TP-/-UP-/- mice will be administered thymidine to enhance the mitochondrial abnormalities. Aim 2: TP-/-UP-/- mice will undergo bone marrow transplantation before and after onset of leukoencephalopathy. Aim 3: TP encapulated in nanoparticles will be administered to TP-/-UP-/- mice.